Method of making a gallium nitride device

ABSTRACT

A method of making a GaN device includes: forming a GaN substrate; forming a plurality of spaced-apart first metal contacts directly on the GaN substrate; forming a layer of insulating GaN on the exposed portions of the upper surface; forming a stressor layer on the contacts and the layer of insulating GaN; forming a handle substrate on the first surface of the stressor layer; spalling the GaN substrate that is located beneath the stressor layer to separate a layer of GaN and removing the handle substrate; bonding the stressor layer to a thermally conductive substrate; forming a plurality of vertical channels through the GaN to define a plurality of device structures; removing the exposed portions of the layer of insulating GaN to electrically isolate the device structures; forming an ohmic contact layer on the second surface; and forming second metal contacts on the ohmic contact layer.

DOMESTIC PRIORITY

This application is a divisional of U.S. application Ser. No.15/630,102, titled “METHOD OF MAKING A GALLIUM NITRIDE DEVICE” filedJun. 22, 2017, which is a divisional of U.S. application Ser. No.14/986,519, titled “METHOD OF MAKING A GALLIUM NITRIDE DEVICE” filedDec. 31, 2015, the contents of which are incorporated by referenceherein in their entirety.

BACKGROUND

The present invention generally relates to semiconductor devicemanufacturing and semiconductor devices, and more particularly to amethod of making gallium nitride (GaN) devices by spalling and the GaNdevices made thereby.

Surface layer removal from brittle substrates using controlled spallingtechnology is a powerful method for changing the cost structure ofhigh-efficiency photovoltaic materials, as well as enabling new featuresin a range of semiconductor technologies (e.g., flexible photovoltaics,flex circuits and displays). This technology uses application of atensile stressor layer on the surface of a base substrate to be spalled.The tensile stressor layer has a combined thickness and stress that issufficient to induce spalling mode fracture in the base substrate.

Handling layers that are applied to the surface of thestressor/substrate combination are then used to control the initiationand fracture propagation (spalling) leading to the removal of continuoussurface layers from the base substrate.

Spalling offers a low cost, simple approach for removing many thinsemiconductor layers from a comparatively expensive thick base substrateof many different semiconductor materials. The thin layers ofsemiconductor material can then be used for semiconductor devicemanufacture. This method enables the low cost manufacture of novel thinlayers of semiconductor materials that have not previously beenavailable for device manufacture in this form. These newly availablethin layers of semiconductor materials require new processes for themanufacture of semiconductor devices from them.

The method described herein advantageously provides a method for themanufacture of GaN devices from a thin layer of spalled GaN,particularly novel GaN devices comprising an insulating GaN layer formedfrom the spalled GaN. The method also includes a method a forming theGaN insulating layer.

SUMMARY

According to an embodiment of the present invention, a method of makinga GaN device is disclosed. The method includes: forming a GaN substratehaving an upper surface; forming a plurality of spaced-apart first metalcontacts directly on the upper surface of the GaN substrate, thecontacts spaced apart by exposed portions of the upper surface; forminga layer of insulating GaN on the exposed portions of the upper surface;forming a stressor layer on the contacts and the layer of insulatingGaN, the stressor layer having a first surface; forming a handlesubstrate on the first surface of the stressor layer; spalling the GaNsubstrate that is located beneath the stressor layer to separate a layerof GaN and expose a stressor layer second surface and removing thehandle substrate; bonding the first surface of the stressor layer to athermally conductive substrate; forming a plurality of vertical channelshaving opposing sidewalls through the insulating GaN to provide exposedportions of the layer of insulating GaN to define a plurality of devicestructures, each device structure comprising at least one contact;removing the exposed portions of the layer of insulating GaN toelectrically isolate the device structures; and forming an ohmic contactlayer on the stressor layer second surface; and forming a plurality ofsecond metal contacts on the ohmic contact layer.

According to another embodiment of the present invention, a secondmethod of making a GaN device is disclosed. The method includes: forminga GaN substrate having an upper surface; forming a plurality ofspaced-apart first metal contacts directly on the upper surface of theGaN substrate, the contacts spaced apart by exposed portions of theupper surface; forming a plurality of first vertical channels havingfirst opposing sidewalls in the exposed portions of the upper surfacepartially through the insulating GaN to define a plurality of devicestructures, each device structure configured to comprise at least onecontact; forming a metal layer on the contacts, the exposed portions ofthe upper surface and the first opposing sidewalls, the forming of themetal layer forming a layer of insulating GaN on the exposed portions ofthe upper surface and the first opposing sidewalls; forming a stressorlayer on the metal layer, the stressor layer having a first surface;forming a handle substrate on the first surface of the stressor layer;spalling the GaN substrate that is located beneath the stressor layer toseparate a layer of GaN and expose a stressor layer second surface andremoving the handle substrate; bonding the first surface of the stressorlayer to a thermally conductive substrate; defining a plurality ofsecond vertical channels having second opposing sidewalls andcorresponding to the first vertical channels through the insulating GaNto exposed portions of the metal layer in the first channels; removingthe metal layer in and under the first channels to the stressor layer toelectrically isolate the device structures; forming an ohmic contactlayer on the stressor layer second surface; and forming a plurality ofsecond metal contacts on the ohmic contact layer.

According to yet another embodiment of the present invention, asemiconductor device includes a substrate, a first insulating layerarranged on the substrate, a conductive metal layer arranged on thefirst insulating layer, a contact arranged on the conductive metallayer, a semiconductor layer arranged on the contact, and an oxideregion arranged adjacent to the contact in the conductive metal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of an embodiment of forming aGaN substrate according to a first embodiment of a method of making aGaN device;

FIG. 2 is a cross-sectional illustration of an embodiment of forming aplurality of spaced-apart first metal contacts according to a firstembodiment of a method of making a GaN device;

FIG. 3 is a cross-sectional illustration of an embodiment of forming alayer of insulating GaN according to a first embodiment of a method ofmaking a GaN device;

FIG. 4 is a cross-sectional illustration of an embodiment of forming astressor layer according to a first embodiment of a method of making aGaN device;

FIG. 5 is a cross-sectional illustration of an embodiment of forming ahandle substrate according to a first embodiment of a method of making aGaN device;

FIG. 6 is a cross-sectional illustration of an embodiment of spallingthe GaN substrate according to a first embodiment of a method of makinga GaN device;

FIG. 7 is a cross-sectional illustration of an embodiment of bonding thefirst surface of the stressor layer to a thermally conductive substrateaccording to a first embodiment of a method of making a GaN device;

FIG. 8 is a cross-sectional illustration of an embodiment of forming aplurality of channels in the insulating GaN layer according to a firstembodiment of a method of making a GaN device;

FIG. 9 is a cross-sectional illustration of an embodiment of removingthe exposed portions of the layer of insulating GaN to electricallyisolate the device structures according to a first embodiment of amethod of making a GaN device;

FIG. 10 is a cross-sectional illustration of an embodiment of forming anohmic contact layer on the stressor layer second surface according to afirst embodiment of a method of making a GaN device;

FIG. 11 is a cross-sectional illustration of an embodiment of forming aplurality of second metal contacts on the ohmic contact layer accordingto a first embodiment of a method of making a GaN device and anembodiment of the structure of the GaN device made thereby;

FIG. 12 is a cross-sectional illustration of an embodiment of forming aGaN substrate according to a second embodiment of a method of making aGaN device;

FIG. 13 is a cross-sectional illustration of an embodiment of forming aplurality of spaced-apart first metal contacts according to a secondembodiment of a method of making a GaN device;

FIG. 14 is a cross-sectional illustration of an embodiment of forming aplurality of channels in the insulating GaN layer according to a secondembodiment of a method of making a GaN device;

FIG. 15 is a cross-sectional illustration of an embodiment of forming ametal layer on the contacts, the exposed portions of the upper surfaceand the first opposing sidewalls, the forming of the metal layer forminga layer of insulating GaN on the exposed portions of the upper surfaceand the first opposing sidewalls according to a second embodiment of amethod of making a GaN device;

FIG. 16 is a cross-sectional illustration of an embodiment of forming astressor layer according to a second embodiment of a method of making aGaN device;

FIG. 17 is a cross-sectional illustration of an embodiment of forming ahandle substrate according to a second embodiment of a method of makinga GaN device;

FIG. 18 is a cross-sectional illustration of an embodiment of spallingthe GaN substrate according to a second embodiment of a method of makinga GaN device;

FIG. 19 is a cross-sectional illustration of an embodiment of bondingthe first surface of the stressor layer to a thermally conductivesubstrate according to a second embodiment of a method of making a GaNdevice;

FIG. 20 is a cross-sectional illustration of an embodiment of defining aplurality of second vertical channels having second opposing sidewallsand corresponding to the first vertical channels through the insulatingGaN layer to exposed portions of the metal layer in the first channelsaccording to a second embodiment of a method of making a GaN device;

FIG. 21 is a cross-sectional illustration of an embodiment of removingthe metal layer in and under the first channels to the stressor layer toelectrically isolate the device structures according to a firstembodiment of a method of making a GaN device;

FIG. 22 is a cross-sectional illustration of an embodiment of forming anohmic contact layer on the stressor layer second surface according to asecond embodiment of a method of making a GaN device;

FIG. 23 is a cross-sectional illustration of an embodiment of forming aplurality of second metal contacts on the ohmic contact layer accordingto a second embodiment of a method of making a GaN device and a secondembodiment of the structure of a GaN device made thereby; and

FIG. 24 is a cross-sectional illustration of an embodiment thatoptionally includes oxidizing the portions of the metal layer under thefirst channels between the GaN layer/first metal contacts and thestressor layer to increase the electrical isolation between the devicestructures according to a first embodiment of a method of making a GaNdevice.

FIG. 25 is a cross-sectional illustration of another alternateembodiment.

DETAILED DESCRIPTION

To achieve these and other advantages, and in accordance with thepurpose of this invention as embodied and broadly described herein, thefollowing detailed embodiments can be embodied in various forms.

The specific processes, materials compounds, compositions, andstructural details set out herein not only comprise a basis for theclaims and a basis for teaching one skilled in the art to employ thepresent invention in any novel and useful way, but also provide adescription of how to make and use this invention.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to illustrate the presentdisclosure. However, it will be appreciated by one of ordinary skill inthe art that various embodiments of the present disclosure may bepracticed without these, or with other, specific details. In otherinstances, well-known structures or processing steps have not beendescribed in detail in order to avoid obscuring the various embodimentsof the present disclosure.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

Referring to the figures, particularly FIGS. 1-25, embodiments of amethod 100, 200 of making gallium nitride (GaN) devices are disclosed.The methods 100, 200 may be advantageously used to make various GaNsemiconductor devices, including various diode and transistor devices,including various light emitting diodes (LED's). GaN is a binary III/Vdirect bandgap semiconductor suitable for use to make brightlight-emitting diodes. GaN is a very hard material that generally has aWurtzite crystal structure, and alternately can be formed with a cubicstructure. It has a wide band gap of about 3.4 eV which affordsapplications in optoelectronic devices, including high-power andhigh-frequency devices. For example, GaN may be used to make violet (405nm) laser diodes, without the use of nonlinear opticalfrequency-doubling. The GaN devices afforded by the method 100, 200 havelow sensitivity to ionizing radiation, making it a suitable material forsolar cell arrays for satellites. GaN transistors can operate at muchhigher temperatures and work at much higher voltages than galliumarsenide (GaAs) transistors, for example, and are useful for poweramplifiers at microwave frequencies. GaN can be doped with silicon (Si)or with oxygen to n-type and with magnesium (Mg) to p-type. Galliumnitride compounds also tend to have a microstructure that includes ahigh dislocation density, on the order of a hundred million to tenbillion defects per square centimeter, and without being limited bytheory the GaN microstructure is believed to be sensitive to varioussemiconductor processing-induced defects when exposed to varioussemiconductor processes, particularly those utilizing plasma discharge,such as, for example, various sputtering, reactive ion etching, ionmilling processes. The very high breakdown voltages, high electronmobility and saturation velocity of GaN has also made it an idealcandidate for high-power and high-temperature microwave applications, asevidenced by its high Johnson's figure of merit. Additional potentialapplications for high-power/high-frequency devices based on GaN includemicrowave radio-frequency power amplifiers (such as those used inhigh-speed wireless data transmission) and high-voltage switchingdevices for power grids, and as RF transistors, such as the microwavesource for microwave ovens, and in various FET structures. The largeband gap means that the performance of GaN transistors is maintained upto higher temperatures than silicon transistors. The methods 100, 200may be performed in any suitable sequence of the elements/operationsdisclosed herein, including the specific sequences of the elements andoperations described herein.

Referring to FIGS. 1-11, a method 100 of making a GaN device isdisclosed. The method 100 includes: forming 105 a GaN (gallium nitride)substrate having an upper surface; forming 110 a plurality ofspaced-apart first metal contacts directly on the upper surface of theGaN substrate, the first metal contacts spaced apart by exposed portionsof the upper surface; forming 115 a layer of insulating GaN on theexposed portions of the upper surface; forming 120 a stressor layer onthe contacts and the layer of insulating GaN, the stressor layer havinga first surface; forming 125 a handle substrate on the first surface ofthe stressor layer; spalling 130 the GaN substrate that is locatedbeneath the stressor layer to separate layer of GaN and expose astressor layer second surface and removing the handle substrate; bonding135 the first surface of the stressor layer to a thermally conductivesubstrate; forming 140 a plurality of vertical channels having opposingsidewalls through the insulating GaN to provide exposed portions of thelayer of insulating GaN to define a plurality of device structures, eachdevice structure comprising at least one contact; removing 145 theexposed portions of the layer of insulating GaN to electrically isolatethe device structures; forming 150 an ohmic contact layer on thestressor layer second surface; and forming 155 a plurality of secondmetal contacts on the ohmic contact layer. The elements of method 100are explained in greater detail below.

Referring to FIG. 1, the method 100 includes forming 105 a GaN substrate2 having an upper surface 4. The GaN substrate 2 may include anysuitable GaN substrate form, including bulk GaN or GaN that is depositedor grown on a supporting substrate 8. The GaN substrate comprises asemiconductor material, the semiconductor material can be doped, undopedor contain doped regions and undoped regions. The semiconductor materialthat can be employed as the GaN substrate 2 can be single-crystalline(i.e., a material in which the crystal lattice of the entire sample iscontinuous and unbroken to the edges of the sample, with no grainboundaries). In another embodiment, the semiconductor material that canbe employed as the GaN substrate 2 can be polycrystalline (i.e., amaterial that is composed of many crystallites of varying size andorientation; the variation in direction can be random (called randomtexture) or directed, possibly due to growth and processing conditions).In yet another embodiment, the semiconductor material that can beemployed as the GaN substrate 2 can be amorphous (i.e., anon-crystalline material that lacks the long-range order characteristicof a crystal). Typically, the semiconductor material that can beemployed as the GaN substrate 2 is a single-crystalline material.

Alternately, the GaN substrate 2 may be formed or deposited on asuitable supporting substrate 8 of another material, such as a sapphire,silicon carbide, zinc oxide, silicon, ceramic, or glass substrate. Theglass can be an SiO₂-based glass which may be undoped or doped with anappropriate dopant. Examples of SiO₂-based glasses that can be employedas the supporting substrate 8 include undoped silicate glass,borosilicate glass, phosphosilicate glass, fluorosilicate glass, andborophosphosilicate glass. When the supporting substrate 8 comprises aceramic, the ceramic may include various inorganic, nonmetallic solidsuch as, for example, a metallic oxide including, but not limited to,alumina, beryllia, ceria and zirconia, a metallic non-oxide including,but not limited to, a metallic carbide, boride, nitride, or silicide; orcomposites that include combinations of oxides and non-oxides. In someembodiments of the present disclosure, the first or upper surface 4 ofthe GaN substrate 2 can be cleaned prior to further processing to removesurface oxides and/or other organic or inorganic contaminants therefrom.In one embodiment, the substrate 2 is cleaned by applying a solvent tothe upper surface 4, such as, for example, acetone and isopropanol,which is capable of removing certain organic or particulate contaminantsand/or surface oxides from the upper surface 4 of the GaN substrate 2.

Forming 105 the GaN substrate 2 may be accomplished using any suitableprocess or combination of processes to make any suitable form, includingin bulk form as a gallium nitride ingot or boule or crystal, including asingle crystal. Alternately, the GaN substrate 2 may be formed ordeposited on a suitable support substrate 8 of another material, such asa sapphire, silicon carbide, silicon or zinc oxide substrate. The GaNsubstrate may be formed by any suitable process, such as, for example,chemical vapor deposition (e.g. MOCVD), physical vapor deposition,molecular beam epitaxy, thermal cracking, plasma deposition using radiofrequency (RF) plasmas, electron cyclotron resonance plasmas, andKaufman ion source plasmas. The GaN substrate 2 may receive any suitableprocessing and have any suitable dopant structure, including dopants andprocesses that provide a p-type or n-type GaN semiconductor material,particularly dopants and processes that provide conductive p-type GaNsemiconductors. The GaN substrate 2 may have any suitable thickness. Inone embodiment, the thickness will be at least about 25 μm, and moreparticularly at least about 50 μm, and even more particularly at leastabout 100 μm.

Referring to FIG. 2, the method 100 includes forming 110 a plurality ofspaced-apart first metal contacts 6 directly on the upper surface 4 ofthe GaN substrate 2. Any suitable number of first metal contacts 6 maybe formed directly on the upper surface 4. The first metal contacts 6may be formed in any suitable pattern, including as an orthogonal arrayof spaced apart contacts. Adjacent first metal contacts 6 may bespaced-apart by any suitable distance. First contacts 6 may be made fromany suitable electrically and thermally conductive material, includingany suitable metal, such as, for example, pure metals or alloyscomprising pure Ag, Au, Pd, Pt, Ni, W, Cr, Nb, Al, Ta, Cu, Mg, or Mo, oralloys of these metals with themselves (e.g. Ni/Au or Ni/Ag alloys) orwith other metals, or any combination of the aforementioned metals. Inone embodiment, first metal contacts 6 will be a good electrical andthermal conductor, such as pure metals or alloys comprising Al, Cu, Au,Ag, Mo, Zn, or Ni, or a combination thereof. First contact 6 willgenerally extend over a significant portion of the device structure toenable and promote transfer of heat away from the GaN device structure,which in one embodiment will include 90% of the surface area of the GaNdevice with which it is associated, and more particularly 75% of thesurface area, and more particularly about 50% of the surface area. Firstcontact 6 may have any suitable layer thickness, which in one embodimentwill include 0.01 to 500 μm, and more particularly 0.05 to 50 μm, andeven more particularly 0.1 to 50 μm, and still more particularly 0.5 to25 μm. In one embodiment, forming 110 of metal contacts may includedeposition of a metal contact adhesion layer 7 on upper surface 4 priorto deposition of the metal layer used to form first metal contacts 6.The metal contact adhesion layer 7 may include any metal that promotesor enhances the adhesion of the metal layer used to form first metalcontacts 6 on the upper surface 4 of the GaN substrate 2. Example firstmetal contacts 6 that are ohmic to p-type GaN include thin Ni (1 to 5nm) and Au or Ag (5 to 100 nm) bilayers that are subsequently annealedat 350-550° C. in an oxygen-containing environment. Example first metalcontacts 6 that are ohmic to n-type GaN include Ti (50 to 100 nm) and Al(100 to 500 nm) bilayers that are subsequently annealed at 350-550° C.in an inert environment.

Forming 110 a plurality of spaced-apart first metal contacts 6 may beformed using any suitable conventional method for forming metal contactson semiconductor materials, including various additive or subtractivemethods, or combinations thereof. In one example of a subtractivemethod, a metal layer may be deposited directly over the entire uppersurface 4 using conventional metal deposition methods, such assputtering, thermal or electron beam evaporation, plating and the like.The metal layer may then be covered with a layer of photoresist materialand patterned to define portions of the photoresist materialcorresponding to the desired first metal contacts 6 and remove theremainder of the photoresist material. The metal layer may then beetched, using wet chemical etchants or dry etching by sputtering, orother conventional dry etching techniques, to remove the portions of themetal layer that are not covered by photoresist thereby defining firstcontacts 6. The definition of first metal contacts 6 also definesexposed portions of the upper surface 4. Alternatively, forming 110 aplurality of spaced-apart first metal contacts may be formed by applyinga contact mask prior to metal deposition, or by using aphotolithographic lift-off process.

Referring to FIG. 3, the method 100 includes forming 115 a layer ofinsulating GaN 14 on the exposed portions 10 of the upper surface 4. Inone embodiment, forming 115 a layer of insulating GaN 14 on the exposedportions 10 of the upper surface 4 comprises inducing microstructuralchanges in a layer proximate the upper surface 4. In this embodiment,the microstructural changes in the upper surface 4 may be produced byintroducing the exposed portions 10 to a microstructural damage inducingconstituent, such as, for example, exposure to a plasma or other sourceof energetic particles, such as, for example, by plasma etching theupper surface using sputtering, reactive ion etching or other techniquesto bombard the upper surface 4 with highly energetic gas ions (e.g. Argas ions) or other species having an energy sufficient to inducemicrostructural changes (e.g. damage) in the GaN. This may include, forexample, altering the dislocation structure or dislocation density in alayer proximate to upper surface 4 to provide the layer of insulatingGaN 14, such as by increasing the dislocation density. In one embodiment(not shown), deposition of a metal layer, such as a titanium layer, isnot needed to affect creation of the layer of insulating GaN 14 suchthat the stressor layer 18 may be deposited directly onto the exposedportions 10 of the upper surface 4 and the first metal contacts 6. Thenature of the microstructural damage that leads to conversion of the GaNto insulating is not well understood, however the near surfacestoichiometry of the binary alloy appears to be altered during plasmabombardment rendering the surface (up to −20 nm for 300W RF Ar plama) Garich. For p-type GaN, the Mg dopant is deactivated by the presence of H,therefore a two-step conversion may be used wherein plasma damage isintroduced to render the p-GaN surface insulating, then a second step ofexposure to H may be performed to further suppress electricalconduction.

In another embodiment, forming 115 the layer of insulating GaN 14 on theexposed portions 10 of the upper surface 4 comprises depositing 160 ametal layer 16 on the first metal contacts 6 and the exposed portions 10of the upper surface 4, wherein depositing 160 the metal layer 16comprises forming 115 the layer of insulating GaN 14 on the exposedportions 10 of the GaN substrate 2. In this embodiment, the depositionof the metal layer provides the microstructural damage inducingconstituent, such as, for example, exposure to a plasma or other sourceof energetic particles, such as, for example, bombarding the uppersurface 4 with highly energetic gas ions (e.g. Ar gas ions) from theplasma used for sputtering the metal or the sputtered metal atoms thatcollide with the surface during deposition or other species having anenergy sufficient to induce microstructural changes (e.g. damage) in theupper surface 4 of the GaN substrate 2. This may also include, forexample, altering the dislocation structure or dislocation density orstoichiometry in a layer proximate to upper surface 4 to provide thelayer of insulating GaN 14, such as by increasing the dislocationdensity or altering the stoichiometry. In one embodiment, the metallayer 16 is a adhesion promoting metal layer 20 promoting the adhesionof the stressor layer 18 to the exposed portions 10 of the upper surface4 and the first metal contacts 6. Metal layer 16 may include anysuitable metal. Metals suitable for use as adhesion promoting metallayer comprise pure Ti, Cr, Ta, W, Cu, Pt, Al, Au, or Ni, or alloys ofthese metals with themselves (e.g. Ti/W alloys) or with other metals, orany combination of the aforementioned metals. The metal layer 16 oradhesion promoting metal layer 20 may comprise a single layer or it mayinclude a multilayered structure comprising at least two layers ofdifferent metals. The metal layer 16 may also be selected to provide adiffusion barrier to limit or prevent diffusion into one or more of theGaN substrate 2, metal contacts 6, and/or stressor layer 18. The use ofpure Ti and Ti alloys, such as Ti/W alloys, is particularly useful inthe methods 100, 200 disclosed herein because Ti can be used to damagethe GaN and then selectively removed by various forms of etching (e.g.wet chemical and plasma etching) relative to the GaN substrate 2, aswell as the materials used for metal contacts 6 and stressor layer 18,to provide electrical isolation and define the device structuresdescribed herein. In addition, pure Ti and Ti alloys can also be readilyoxidized in situ to provide an electrical insulator and thereby alsoprovide electrical isolation and define the device structures describedherein. The metal layer 16 or adhesion promoting metal layer 20 can beformed at room temperature (15° C.−40° C.) or above. In one embodiment,the metal layer 16 or adhesion promoting metal layer 20 is formed at atemperature which is from 20° C. to 180° C. In another embodiment, themetal layer 16 or adhesion promoting metal layer 20 is formed at atemperature which is from 20° C. to 60° C. The metal layer 16 oradhesion promoting metal layer 20 can be formed utilizing depositiontechniques that are well-known to those skilled in the art. For example,the metal layer 16 or adhesion promoting metal layer 20 can be formed bysputtering, chemical vapor deposition, plasma enhanced chemical vapordeposition, chemical solution deposition, physical vapor deposition, orplating. When sputter deposition is employed for depositing 160, thesputter deposition process may further include an in-situ sputter cleanor etch process before the deposition to promote the creation of thelayer of insulating GaN 14 as described above. Thus, forming 115 thelayer of insulating GaN 14 may include forming 115 that includes onlyexposure to energetic particles (e.g. plasma etching) as describedabove, or that includes only deposition 160 of metal layer 16 asdescribed above, or that involves the use of both processes. Depositing160 the metal layer 16 may include sputter deposition, such as, forexample, by DC, RF, ion beam, ion-assisted, or reactive sputtering, ofthe metal layer 16, including adhesion promoting metal layer 20. Withoutbeing limited by theory, it is believed that the energy of the metalatoms striking the surface during sputtering damages or otherwise altersthe GaN microstructure proximate the upper surface 4, thereby causingthe GaN, particularly conductive p-type GaN, to become nonconductive andelectrically insulating, thereby creating a layer of insulating GaN 14.In one embodiment, this may include the introduction of a Ga-rich regionin the GaN by deposition of the metal atoms onto the upper surface 4,and may result from the impact of the metal atoms themselves, or thecarrier atoms used to create the plasma, with upper surface 4. The metallayer 16 or adhesion promoting metal layer 20 typically has a thicknessof from 5 nm to 1000 nm, and more particularly 50 nm to 500 nm, and evenmore particularly a thickness of from 100 nm to 300 nm, with a thicknessof from 100 nm to 150 nm being more typical. Other thicknesses for themetal layer 16 that are below and/or above the aforementioned thicknessranges can also be employed in the present disclosure.

Referring to FIG. 4, the method 100 includes forming 120 a stressorlayer 22 on the first metal contacts 6 and the layer of insulating GaN14, the stressor layer having a first surface 24. The first surface 24is disposed outwardly away from the GaN substrate 2. The stressor layer22 comprises a metal, including various pure metals and metal alloys. Inone embodiment, the metal may comprise Ni, Ti, Mo, Cr, Fe, or W, or acombination thereof. The stressor layer 22 may be a single metal layer,or it may include a multilayered structure comprising at least twolayers of different metals. The stressor layer 22 may optionally beemployed in conjunction with an edge exclusion material to control thespalling of the GaN substrate 2 that is disposed between the stressorlayer 22 and the GaN substrate 2, and in certain embodiments between thestressor layer 22 and the metal layer 16. Thin substrate fabricationusing stress-induced substrate spalling and the use of an edge exclusionmaterial to control spalling by reducing edge-related substrate breakageduring spalling are disclosed in U.S. Pat. Nos. 8,247,261 and 8,748,296,which are incorporated herein by reference in their entirety.

The stressor layer 22 has a fracture toughness that is greater than thefracture toughness of the GaN substrate 2, and is used to induce astress within the GaN substrate 2 that is sufficient to fracture the GaNmaterial a predetermined distance 12 or depth below the substratesurface. The depth may be selectively controlled to establish apredetermined thickness of the GaN substrate 2. Fracture toughness is aproperty which describes the ability of a material containing a crack toresist fracture. Fracture toughness is denoted K_(IC). The subscript Icdenotes mode I crack opening under a normal tensile stress perpendicularto the crack, and c signifies that it is a critical value. Mode Ifracture toughness is typically the most important value becausespalling mode fracture along a predetermined fracture or spalling path70 (e.g. FIG. 4) usually occurs at a location in the substrate wheremode II stress (shearing) is zero, and mode III stress (tearing) isgenerally absent from the loading conditions. Fracture toughness is aquantitative way of expressing a material's resistance to brittlefracture when a crack is present.

Referring to FIG. 4, there is illustrated the structure of FIG. 3 afterforming stressor layer 22 over the upper surface of metal layer 16, suchas titanium. Alternately, stressor layer 22 may be formed directly overthe metal contacts 6 and insulating GaN layer 14.

As mentioned above, stressor layer 22 is located atop the upper surface4 of GaN substrate 2. In one embodiment and when no metal layer 16 oradhesion promoting metal layer 20 is present, the stressor layer 22 isin direct contact with the metal layer 16. In another embodiment andwhen a metal layer 16 is present, the stressor layer 22 is in directcontact with the upper surface of the metal or metallic adhesion layer20.

In accordance with the present disclosure, the stressor layer 22 that isformed atop upper surface 4 of the GaN substrate 2 has a criticalthickness and stress value that cause spalling mode fracture to occurwithin the GaN substrate 2. By “spalling mode fracture” it is meant thata crack is formed within the GaN substrate 2 and the combination ofloading forces maintain a crack trajectory at a depth below thestressor/substrate interface. By critical condition, it is meant thatfor a given stressor material and substrate material combination, athickness value and a stressor value for the stressor layer is chosenthat render spalling mode fracture possible (can produce a K_(I) valuegreater than the K_(IC) of the substrate).

Specifically, the thickness of the stressor layer 22 is chosen toprovide the desired predetermine fracture or spalling path 70 atpredetermined fracture distance 12 or depth within the GaN substrate 2.For example, if the stressor layer 22 is chosen to be Ni, then fracturewill occur at a depth below the stressor layer 22 roughly 2 to 3 timesthe Ni thickness. The stress value for the stressor layer 22 is thenselected to satisfy the critical condition for spalling mode fracture.This can be estimated by inverting the empirical equation given byt*={(2.5×10⁶(K_(Ic) ^(3/2))]/σ², where t* is the critical stressor layerthickness (in microns), K_(Ic) is the fracture toughness (in units ofMPa·m^(1/2)) of the base substrate and σ is the stress value of thestressor layer (in MPa or megapascals). The above expression is a guide,in practice, spalling can occur at stress or thickness values up to 20%less than that predicted by the above expression. In the case ofspalling GaN layers formed on a different substrate, such as SiC orsapphire, the depth of spalling may be determined by the thickness ofthe epitaxially grown GaN layer. In this case, there may be sufficientenergy to fracture GaN, but not the underlying sapphire or SiC which hasa higher fracture toughness. As a result, fracture typically proceedsalong the GaN/substrate growth interface.

In accordance with the present disclosure, the stressor layer 22 isunder tensile stress while present on the GaN substrate 2 at thespalling temperature. The stressor layer 22 may comprise a singlestressor layer, or a multilayered stressor structure including at leasttwo layers of different stressor material can be employed.

In one embodiment, the stressor layer 22 comprises a metal. Stressorlayer 22 may include any metal suitable for stressing the GaN substratesufficient to cause spalling, and in one embodiment, may include, forexample, Ni, Ti, Mo, Cr, Fe, or W. Alloys of these metals can also beemployed. In one embodiment, the stressor layer 22 includes at least onelayer consisting of Ni. In one embodiment, the stressor layer 22employed in the present disclosure is formed at a temperature which isat room temperature (15° C.−40° C.). The stressor layer 22 may have anysuitable thickness, including a thickness of from 3 μm to 50 μm, with athickness of from 5 μm to 35 μm being more typical. Other thicknessesfor a metallic stressor layer that are below and/or above theaforementioned thickness ranges can also be employed in the presentdisclosure.

The method 100 also includes forming 125 a handle substrate 26 on thefirst surface 24 of the stressor layer 22. Referring to FIG. 5, there isillustrated the structure of FIG. 4 after forming 125 a handle substrate26 on the stressor layer 22. The handle substrate 26 employed in thepresent disclosure comprises any flexible material which has a minimumradius of curvature of less than 30 cm. Illustrative examples offlexible materials that can be employed as the handle substrate 26include a metal foil or a polyimide foil. Other examples of flexiblematerials that can be employed as the optional handle substrate 26include polymers, tapes and spin-on materials.

The handle substrate 26 can be used to provide better fracture controland more versatility in handling the spalled portion 28 of the GaNsubstrate 2. Moreover, the handle substrate 26 can be used to guide thecrack propagation during the spalling process of the present disclosure.The handle substrate 26 of the present disclosure is typically, but notnecessarily, formed at room temperature (15° C.−40° C.).

The handle substrate 26 can be formed utilizing deposition techniquesthat are well-known to those skilled in the art including, for example,dip coating, spin-coating, brush coating, sputtering, chemical vapordeposition, plasma enhanced chemical vapor deposition, chemical solutiondeposition, physical vapor deposition, or plating, or a combinationthereof. Handle substrate 26 may also be formed as a stand-alone film ortape, such as a polymer tape (e.g. polyimide, polyester, orpolypropylene) with an integral adhesive layer and forming 125 mayinclude application by pressing the handle substrate 26 against thestressor layer 22 to adhere the adhesive layer. Alternately a separateadhesive may be applied to the stressor layer 22 or handle layer 26 andforming 125 may include application by pressing the handle substrate 26against the stressor layer 22 with the adhesive disposed between them.

The handle substrate 26 may have any suitable thickness. In oneembodiment, handle substrate 26 has a thickness of from 1 μm to 3 mm,and more particularly a thickness of from 25 μm to 120 μm. Otherthicknesses for the handle substrate 26 that are below and/or above theaforementioned thickness ranges can also be employed in the presentdisclosure.

The method 100 also includes spalling 130 the GaN substrate 2 that islocated beneath the stressor layer 22 to separate a GaN layer 30 asspalled portion 28 and expose a stressor layer second surface 32 asillustrated in FIG. 6 and removing the handle substrate 26. Referring toFIG. 6, there is illustrated the structure of FIG. 5 after spalling 130in which a spalled portion 28 of the GaN substrate is removed from theinitial GaN substrate 2 as the layer of GaN 30 having a predeterminedthickness. The portion of the GaN substrate 28 that is removed from theinitial GaN substrate 2 is referred to herein as spalled portion 28; theremaining portion of the GaN substrate is designated as 2′ in FIG. 6.

The spalling process can be initiated at room temperature or at atemperature that is less than room temperature. In one embodiment,spalling 130 is performed at room temperature (i.e., 20° C. to 40° C.).In another embodiment, spalling 130 is performed at a temperature lessthan 20° C. In a further embodiment, spalling occurs at a temperature of77° K or less. In an even further embodiment, spalling 130 occurs at atemperature of less than 206° K. In still yet another embodiment,spalling 130 occurs at a temperature from 175° K to 130° K.

When a temperature that is less than room temperature is used, the lessthan room temperature spalling 130 process can be achieved by coolingthe structure down below room temperature utilizing any cooling means.For example, cooling can be achieved by placing the structure in aliquid nitrogen bath, a liquid helium bath, an ice bath, a dry ice bath,a supercritical fluid bath, or any cryogenic environment liquid or gas.

When spalling 130 is performed at a temperature that is below roomtemperature, the spalled portion 28 is returned to room temperature byallowing the spalled portion 28 structure to slowly warm up to roomtemperature by allowing the same to stand at room temperature.Alternatively, the spalled portion 28 can be heated up to roomtemperature utilizing any heating means.

Referring to FIG. 7, there is illustrated the structure of FIG. 6 afterremoving the handle substrate 26 and inverting the remaining portion ofthe device structure. The handle substrate 26 can be removed from thespalled portion 28 of the GaN substrate 2 utilizing conventionaltechniques well known to those skilled in the art. For example, and inone embodiment, aqua regia (HNO₃/HCl) can be used for removing thehandle substrate 26 from the spalled portion 28 of the GaN substrate 2.In another example, UV or heat treatment is used to remove the handlesubstrate 26.

The thickness of the spalled portion 28 of the GaN substrate 2 shown inFIG. 7 varies depending on the material of the stressor layer 22 and theGaN substrate 2. In one embodiment, the spalled portion 28 of the GaNsubstrate 2 has a thickness of less than 100 microns. In anotherembodiment, the spalled portion 28 of the GaN substrate 2 has athickness of less than 50 microns, and more particularly less than 30microns.

Referring to FIG. 8, the method 100 also includes bonding 135 the firstsurface 24 of the stressor layer 22 to a thermally conductive substrate34. The thermally conductive substrate 34 may be used to remove heatfrom the device structure 36 during operation. In one embodiment, thesubstrate 34 may include any suitable thermally conductive material,including various metals, thermally conductive polymers, thermallyconductive ceramics, or thermally conductive cermets, diamond orconductive diamond like carbon, or a combination thereof. Any suitablemetal may be used. In an embodiment, metals may include Fe, Cu, Al, Ni,or alloys thereof. In an embodiment, thermally conductive polymers mayinclude various filled or unfilled polymers, including metal-filledpolymers, carbon-filled polymers, or polymers filled with variousthermally conductive compounds, including various ceramic materials,such as epoxies that are filled with conductive AlN particles. Ceramicsmay comprise sapphire (Al₂O₃), SiC, and AlN. The thermally conductivesubstrate 34 may have any suitable size and/or thickness necessary toincorporate the device structure 36 into a GaN device, including theexemplary electronic devices described herein. The substrate 34 mayinclude a single material or layer of material, or a multilayermaterial, including a multilayer laminate, such as a conventionalprinted circuit or printed wiring board (e.g. a Cu, glass, polymer resinlaminate). In one embodiment, the substrate 34 may include a multilayermetal/ceramic substrate (e.g. Cu or Au/alumina (sapphire). The substrate34 may have a size and/or thickness necessary to incorporate a singledevice structure 36 or a plurality of device structures 36, including anM×N array that includes any number of device structures 36, such as maybe used to develop an LED display. Bonding 135 may be accomplished usingany suitable bonding material 42 or method of bonding to form ametallurgical bond 80. In one embodiment, the bonding material 42includes a solder layer 44 having a lower melting point than thesubstrate 34 and stressor layer 22 and the solder is melted, reflowedbetween substrate 34 and stressor layer 22, and solidified to join them.In one embodiment, the solder 44 may include a NiAu or a NiAg solder,for example.

The method 100 also includes forming 140 a plurality of verticalchannels 38 having opposing sidewalls 40 from the stressor layer secondsurface 32 through the insulating GaN layer 30 to provide exposedportions 46 of the layer of insulating GaN 14 to define a plurality ofdevice structures 36, each device structure 36 comprising at least onefirst metal contact 6. Forming 140 a plurality of vertical channels 38can be achieved by lithography and etching. Lithography includesapplying a layer of photoresist (not shown) atop the GaN layer 30,exposing the photoresist to a desired pattern of radiation anddeveloping the exposed photoresist utilizing a conventional photoresistdeveloper to selectively open areas of the photoresist over the desiredvertical channels 38 and leave the layer of photoresist in other areas.Other patterning methods may also be employed, including the applicationof various metal masks. The patterned GaN layer 30 may then be etched inthe exposed regions to define the vertical channels 38. Etching mayinclude any suitable form of etching, including various wet and dryetching methods. Wet etching may include, for example, techniquesincluding photo-enhanced electrochemical (PEC), wet etch, photo-assistedcryogenic etch (PAC), crystallographic wet etch, photo-assisted anodicetch, wet chemical digital etch of GaN at room temperature, and a PECbinary etch using K₂S₂O₈ and KOH, and may employ acid and base etchants,including KOH, NaOH, HCl, H₃PO₄, citric acid/H₂O₂, tartaricacid/ethylene glycol, and K₂S₂O₈/KOH, for example. Dry etchingtechniques may include sputtering etching, reactive ion etching, ionbeam etching, plasma etching or laser ablation, for example. Thevertical channels 38 may have any suitable width between opposingsidewalls 40 and channel length. In one embodiment, the channel widthbetween opposing sidewalls 40 is 5 to 1000 μm, and more particularly 10to 300 μm. The vertical channel 38 depth extends from the stressor layersecond surface 32 through the insulating GaN layer 30 to provide exposedportions 46 of the layer of insulating GaN 14, which is substantiallythe entire thickness of the GaN layer 30.

The method 100 further includes removing 145 the exposed portions 46 ofthe layer of insulating GaN 14 to electrically and thermally isolate thedevice structures 36 from one another. In one embodiment, removing 145the exposed portions 46 of the layer of insulating GaN 14 may includeextension of forming 140, such as by using the same etching processselected and used for forming 140 as described above, to also remove theexposed portions 46 of the layer of insulating GaN 14. This may beemployed, for example, where the GaN layer 30 and the exposed portions46 of the layer of insulating GaN 14 have similar etching rates in theetchant that is employed. In other embodiments, it may be desirable toemploy an etchant for removing 145 that is different than the etchantused for forming 140. This may be employed, for example, where the GaNlayer 30 and the exposed portions 46 of the layer of insulating GaN 14have different etching characteristics (e.g. etching rate) in theetchant that is used for forming 140, such that it is desirable toselect a different etchant for removing 145 that has improvedperformance in the layer of insulating GaN 14 versus the GaN layer 30,such as a higher etching rate, improved isotropic or anisotropic etchingperformance (i.e. enhanced or reduced undercut), or etch selectivityversus the underlying material, which may be the stressor layer 22 orthe metal layer 16.

The method 100 also includes forming 150 an ohmic contact layer 48 onthe stressor layer second surface 32. Any suitable low resistance ohmiccontact may be used to allow charge to flow easily in both directionsbetween the GaN layer 30 and second metal contacts 50, without blockingdue to rectification or excess power dissipation due to voltagethresholds. The layer may be deposited and formed by sputtering,chemical vapor deposition, plasma enhanced chemical vapor deposition,chemical solution deposition, physical vapor deposition, or plating, ora combination thereof. Any suitable material that provides an ohmiccontact 50 may be used. In one embodiment, the ohmic contact 50comprises indium tin oxide, aluminum-doped zinc oxide, indium-dopedcadmium oxide, a transparent conductive polymer, or carbon nanotubes, ora combination of the aforementioned materials.

The method 100 also includes forming 155 a plurality of second metalcontacts 48 on the ohmic contact layer 50. The second metal contacts 48may be formed from a metal layer (not shown) that is deposited andformed by any suitable metal layer deposition method, includingsputtering, chemical vapor deposition, plasma enhanced chemical vapordeposition, chemical solution deposition, physical vapor deposition, orplating, or a combination thereof. Any suitable material that may beused to provide a second metal contact 48 may be used. In oneembodiment, the second metal contact 48 may include pure Ag, Au, Pd, Pt,Ni, W, Cr, Nb, Al, Ta, Cu, Mg, or Mo, or alloys of these metals withthemselves (e.g. Ni/Al or Ti/A1 alloys) or with other metals, or anycombination of the aforementioned metals. Desirable second metalcontacts 48 are good electrical conductors that are resistant toenvironmental degradation, such as corrosion or oxidation. In oneembodiment, forming 155 a plurality of second metal contacts 48 on theohmic contact layer 50 can be achieved by lithography and etching of themetal layer. Lithography includes applying a layer of photoresist (notshown) atop the metal layer, exposing the photoresist to a desiredpattern of radiation and developing the exposed photoresist utilizing aconventional photoresist developer to selectively open areas of thephotoresist over the desired second metal contacts 48 and leave thelayer of photoresist in other areas. Other patterning methods may alsobe employed, including the application of various metal (shadow) masks.The metal layer may then be etched in the exposed regions to define thesecond metal contacts 48. Etching may include any suitable form ofetching, including various wet and dry etching methods. The secondelectrical contacts 48 are configured for attachment of electricalinterconnections to electrical circuits that utilize the devicestructures 36.

In one embodiment, method 100 may optionally further comprise etching165 to remove the insulating GaN layer 30 proximate the channels 38 anddefine a final channel width. Etching 165 to remove the insulating GaNlayer 30 may be performed using the etching processes described aboveregarding forming 140 the plurality of vertical channels 38.

In an embodiment, method 100 may also optionally include doping 170 theGaN layer 30 to form a p-type region or an n-type region therein, or acombination thereof. Doping 170 may be performed after spalling 130 theGaN substrate 2 to separate the GaN layer 30 as spalled portion 28 andexpose the a stressor layer second surface 32. N-type doping can beachieved by adding elements such as, for example, silicon or germanium.P-type doping may be achieved, for example, by adding Mg. Doping 170 maybe performed by any suitable method of doping, including ionimplantation.

While many combinations of the materials described above arecontemplated, in one embodiment GaN layer 30 comprises a GaN LEDstructure including p, n, and multi quantum well regions, the metallayer 16 comprises titanium, the stressor layer comprises nickel, thefirst metal contacts comprise nickel and silver, and the ohmic contactscomprise a layer of an optically transparent, electrical conductivematerial comprising indium tin oxide, aluminum-doped zinc oxide,indium-doped cadmium oxide, a transparent conductive polymer, or carbonnanotubes.

In one embodiment, method 100 may optionally include forming a layer ofinsulating GaN 52 (not shown) on the opposing sidewalls 40 of thechannels 38 of the embodiment of FIG. 11. Forming 175 the layer ofinsulating GaN 52 on the opposing sidewalls 40 may be performed usingthe methods described herein for forming the layer of insulating GaN 14.The thickness of the layer of insulating GaN 52 may be any suitablethickness, including those described herein for the layer of insulatingGaN 14.

In one embodiment, method 100 may optionally include etching thestressor layer second surface 32 of the GaN layer 30 prior to formingthe ohmic contact layer 50. Etching 180 may be employed to remove damagein the microstructure of the GaN layer 30 proximate the stressor layersecond surface 32 resulting from spalling 130, for example. Etching 180may be performed using the etching processes described above regardingforming 140 the plurality of vertical channels 38, and may be performedconcurrently with forming 140 or etching 165, for example, or may beperformed as a separate step.

In one embodiment, a second method 200 of making a GaN device isdisclosed. Method 200 uses a number of operations that are the same orsimilar to those of method 100 and incorporates various elements,structures, and features that are common to those of the GaN device ofmethod 100. Thus, references below to various elements, structures, andfeatures employ the same reference numerals. Similarly, the operationsof method 200 are explained in a number of instances below usingcomparative references to the operations of method 100 for brevity.

Referring to FIGS. 12-25, in one embodiment, method 200 includes:forming 205 a GaN substrate 2 having an upper surface 4; forming 210 aplurality of spaced-apart first metal contacts 6 directly on the uppersurface 4 of the GaN substrate 2, the contacts spaced apart by exposedportions 10 of the upper surface 4; forming 215 a plurality of firstvertical channels 38 having first opposing sidewalls 40 in the exposedportions 10 of the upper surface 4 partially through the GaN substrate 2to define a plurality of device structures 36, each device structureconfigured to comprise at least one contact 6; forming 220 a metal layer16 on the contacts 6, the exposed portions 10 of the upper surface 4 andthe first opposing sidewalls 40, the forming of the metal layer forminga layer of insulating GaN 14, 52 on the exposed portions 10 of the uppersurface and the first opposing sidewalls 40 (and channel bottom);forming 225 a stressor layer 22 on the metal layer 16, the stressorlayer 22 having a first surface 24; forming 230 a handle substrate 26 onthe first surface 24 of the stressor layer 22; spalling 235 the GaNsubstrate 2 that is located beneath the stressor layer 22 to separate alayer of GaN 30 and expose a stressor layer second surface 32 andremoving the handle substrate 26; bonding 240 the first surface 24 ofthe stressor layer 22 to a thermally conductive substrate 34; defining245 a plurality of second vertical channels 56 having second opposingsidewalls 58 and corresponding to the first vertical channels 38 throughthe insulating GaN layer 30 to exposed portions 60 of the metal layer 16in the first channels 38; removing 250 the metal layer 16 in and underthe first channels 38 to the stressor layer 22 to electrically isolatethe device structures 36; and forming 255 an ohmic contact layer 50 onthe stressor layer second surface 32; and forming 260 a plurality ofsecond metal contacts 48 on the ohmic contact layer 50.

Referring to FIG. 12, in one embodiment, method 200 includes forming 205the GaN substrate 2 having an upper surface 4. Forming 205 may beaccomplished in the same manner as forming 105, described above.

Referring to FIG. 13, in one embodiment, method 200 includes forming 210a plurality of spaced-apart first metal contacts 6 directly on the uppersurface 4 of the GaN substrate 2, the contacts spaced apart by exposedportions 10 of the upper surface 4. Forming 210 may be accomplished inthe same manner as forming 110, described above.

Referring to FIG. 14, in one embodiment, method 200 includes forming 215a plurality of first vertical channels 38 having first opposingsidewalls 40 in the exposed portions 10 of the upper surface 4 partiallythrough the GaN substrate 2 to define a plurality of device structures36, each device structure configured to comprise at least one contact 6.Forming 215 may be accomplished in the same manner as forming 140, asdescribed above. However, in the case of forming 215, in one embodimentthe vertical channels 38 extend only partially through the insulatingGaN layer 30 to a predetermined depth, and do not extend entirelythrough the insulating GaN layer so that the channels 38 are notinvolved in the predetermined fracture or spalling path 70 duringspalling.

Referring to FIG. 15, in one embodiment, method 200 includes forming 220a metal layer 16 on the contacts 6, the exposed portions 10 of the uppersurface 4 and the first opposing sidewalls 40, the forming of the metallayer forming a layer of insulating GaN 14 on the exposed portions 10 ofthe upper surface and a layer of GaN 52 on the first opposing sidewalls40 and bottom of the channel 41. Forming 220 may be accomplished in thesame manner as forming 115, 160, as described above.

Referring to FIG. 16, in one embodiment, method 200 includes forming 225a stressor layer 22 on the metal layer 16, the stressor layer 22 havinga first surface 24; forming 230 a handle substrate 26 on the firstsurface 24 of the stressor layer 22. Forming 225 may be accomplished inthe same manner as forming 120, as described above.

Referring to FIG. 17, in one embodiment, method 200 includes forming 230a handle substrate 26 on the first surface 24 of the stressor layer 22.Forming 230 may be accomplished in the same manner as forming 125, asdescribed above.

Referring to FIG. 18, in one embodiment, method 200 includes spalling235 the GaN substrate 2 that is located beneath the stressor layer 22 toseparate a layer of GaN 30 and expose a stressor layer second surface 32and removing the handle substrate 26. Spalling 235 may be accomplishedin the same manner as spalling 130, as described above.

Referring to FIG. 19, in one embodiment, method 200 includes bonding 240the first surface 24 of the stressor layer 22 to a thermally conductivesubstrate 34. Bonding 240 may be accomplished in the same manner asbonding 135, as described above.

Referring to FIG. 20, in one embodiment; method 200 includes defining245 a plurality of second vertical channels 56 having second opposingsidewalls 58 and corresponding to the first vertical channels 38 throughthe insulating GaN layer 30 to exposed portions 60 of the metal layer 16in the first vertical channels 38. Defining 245 may be accomplished inthe same manner as forming 140, as described above. However, in the caseof defining 245 the openings in the photoresist must be opened over theportions of the surface directly above and corresponding to the firstvertical channels as would be readily understood by one of ordinaryskill in the art.

Referring to FIG. 21, in one embodiment, method 200 includes removing250 the metal layer 16 in and under the first channels 38 to thestressor layer 22 to electrically isolate the device structures 36; andforming 255 an ohmic contact layer 50 on the stressor layer secondsurface 32; and forming 260 a plurality of second metal contacts 48 onthe ohmic contact layer 50. Removing 250 may be performed by anysuitable removal method. In one embodiment, removing 250 includesetching. Etching may include any suitable form of etching, includingvarious wet chemical and dry etching methods, including sputteringetching, reactive ion etching, ion beam etching, plasma etching or laserablation, for example.

Referring to FIG. 22, in one embodiment, method 200 includes forming 255an ohmic contact layer 50 on the stressor layer second surface 32.Forming 255 may be accomplished in the same manner as forming 150, asdescribed above.

Referring to FIG. 23, in one embodiment, method 200 includes forming 260a plurality of second metal contacts 48 on the ohmic contact layer 50.Forming 260 may be accomplished in the same manner as forming 155, asdescribed above.

In one embodiment, method 200 may optionally further comprise etching toremove the insulating GaN layer 30 proximate the channels 38 and definea final channel width. Etching to remove the insulating GaN layer 30 maybe performed using the etching processes described above regardingforming 140 the plurality of vertical channels 38.

In one embodiment, method 200 may also optionally include doping theinsulating GaN layer 30 to form a p-type region or an n-type regiontherein, or a combination thereof. Doping may be performed afterspalling 130 the GaN substrate 2 to separate the GaN layer 30 as spalledportion 28 and expose the a stressor layer second surface 32. N-typedoping can be achieved by adding elements such as, for example, siliconor germanium. P-type doping may be achieved, for example, by adding Mg.Doping may be performed by any suitable method of doping, including ionimplantation.

In one embodiment, method 200 may optionally include forming a layer ofinsulating GaN 52 on the opposing sidewalls 40 of the channels 38, asillustrated in FIG. 20. Forming the layer of insulating GaN 52 on theopposing sidewalls 40 may be performed using the methods describedherein for forming the layer of insulating GaN 14. The thickness of thelayer of insulating GaN 52 may be any suitable thickness, includingthose described herein for the layer of insulating GaN 14.

In one embodiment, method 200 may optionally include etching thestressor layer second surface 32 of the GaN layer 30 prior to formingthe ohmic contact layer 50 as illustrated in FIG. 7. Etching 280 may beemployed to remove damage in the microstructure of the GaN layer 30proximate the stressor layer second surface 32 resulting from spalling130, for example. Etching may be performed using the etching processesdescribed above regarding forming 215. The plurality of verticalchannels 38, and may be performed concurrently with forming 215 oretching, for example, or may be performed as a separate step.

Referring to FIG. 24, in an alternate embodiment of method 200,following defining 245, removing 250 is omitted and replaced withoxidizing 290 the metal layer 16 in and under the first channels 38entirely through the metal layer 16 to the stressor layer 22 toelectrically isolate the device structures 36 by forming an oxide plug68 in the channel. All other operations of method 200 remain the same.Thus, rather than removing the metal layer 16 in the vertical channels38, the metal layer 16 is converted to an insulating metal oxide in theform of oxide plug 68. Oxidizing 290 may be performed by any suitablemethod of oxidizing the metal layer, including heating the devicestructure in oxygen or another suitable oxidizing gas using conventionalmethods of semiconductor processing to convert a metal or metallic layer(e.g. Si) to a metal oxide.

Referring to FIG. 25, in one embodiment, method 200 may also optionallyinclude oxidizing 280 the portions 64 of the metal layer 16 under thefirst channels 38 between the GaN layer 30/first metal contacts 6 andthe stressor layer 22 to increase the electrical isolation between thedevice structures 36. Oxidizing 280 the portions 64 of the metal layer 1may be accomplished using thermal (gaseous) or anodic (solution)oxidation. In one embodiment, the oxidized portions 64 of the metallayer 16 comprise an oxide ring.

Methods 100 and 200 advantageously enable the development of novel GaNdevices as illustrated in FIGS. 11, 23-25. The GaN devices may be usedas diodes, including LED's, and transistor structures, including MESFETstructures.

The various numerical ranges describing the invention as set forththroughout the specification also includes any combination of the lowerends of the ranges with the higher ends of the ranges, and any singlenumerical value, or any single numerical value that will reduce thescope of the lower limits of the range or the scope of the higher limitsof the range, and also includes ranges falling within any of theseranges.

The terms “about,” “substantial,” or “substantially” in any claim or asapplied to any parameters herein, such as a numerical value, includingvalues used to describe numerical ranges, means slight variations in theparameter. In another embodiment, the terms “about,” “substantial,” or“substantially,” when employed to define numerical parameter include,e.g., a variation up to five per-cent, ten per-cent, or 15 per-cent, orsomewhat higher or lower than the upper limit of five per-cent, tenper-cent, or 15 per-cent. The term “up to” that defines numericalparameters means a lower limit comprising zero or a miniscule number,e.g., 0.001. The terms “about,” “substantial” and “substantially” alsomean that which is largely or for the most part or entirely specified.The inventors also employ the terms “substantial,” “substantially,” and“about” in the same way as a person with ordinary skill in the art wouldunderstand them or employ them. The phrase “at least” means one or acombination of the elements, materials, compounds, compositions, orconditions, and the like specified herein, wherein “combination” isdefined above. The terms “written description,” “specification,”“claims,” “drawings,” and “abstract” as used herein refer to the writtendescription, specification, claims, drawings, and abstract of thedisclosure as originally filed, or the written description,specification, claims, drawings, and abstract of the disclosure assubsequently amended, as the case may be.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A semiconductor device comprising: a stressedlayer; a conductive metal layer positioned on the stressed layer;contacts separately positioned on the conductive metal layer, theconductive metal layer being on at least a portion of sides of thecontacts; a substrate positioned on the contacts such that an insulatinglayer is arranged on portions of the substrate proximate to thecontacts; and an oxide region arranged adjacent to the contacts so as toprovide a separation between the contacts.
 2. The device of claim 1,further comprising a cavity partially defined by the oxide region. 3.The device of claim 2, wherein the cavity separates the contacts.
 4. Thedevice of claim 1, wherein the oxide region includes TiO_(x).
 5. Thedevice of claim 1, wherein the insulating layer includes GaN.
 6. Thedevice of claim 1, wherein the contacts are selected from the groupconsisting of Ag, Au, Pd, Pt, Ni, W, and Cr.
 7. The device of claim 1,wherein the contacts are selected from the group consisting of Nb, Al,Ta, Cu, Mg, and Mo.
 8. The device of claim 1, wherein the conductivemetal layer is an adhesion promoting metal.
 9. The device of claim 1,wherein the conductive metal layer is selected from the group consistingof Ti, Cr, Ta, W, Cu, Pt, Al, Au, and Ni.